There was a thread about whether it's better to add a bit extra to your primary fermenter, or to top up when you add to secondary. In a midnight moment of clarity, i realized that beer will come out marginally stronger if you add 1 gallon extra to the primary, rather than to the secondary. The gallon has less of a diluting effect in the larger quantity in the primary.

This assumes that you would be adding the same amount of water at either stage, which may or may not be true. However, I will say that I prefer the idea of adding extra water sooner rather than later, if only for the purposes of non-contamination.